The many years' of difficulty with laser fusion owing to the complexity of the interaction with the plasma has been clarified experimentally from the 10-ps pulsation of the interaction. The theoretical understanding of this pulsation is presented here from a hydrodynamic simulation of the interaction in which the generation of partial standing waves and the subsequent nonlinear-force-produced density ripple produces ideal Bragg reflection in the outermost part of the plasma. This all decays within about 10 ps hydrodynamically. The theory explains immediately why the 2-ps coherence at induced spatial incoherence avoids the pulsation and arrives at a smooth interaction similar to the random-phase plate or the use of broad-band laser spectrum irradiation.